When we say that someone has “learned” a subject, we typically mean that they have shown evidence of mastery not only of basic cognitive processes like factual recall and working mechanical exercises but also higher-level tasks like applying concepts to new problems and judging between two equivalent concepts. A student learning calculus, for instance, needs to demonstrate that s/he can do things like take derivatives of polynomials and use the Chain Rule. But if this is all they can demonstrate, then it’s stretching it to say that the student has “learned calculus”, because calculus is a lot more than just executing mechanical processes correctly and quickly…
Khan Academy is great for learning about lots of different subjects. But it’s not really adequate for learning those subjects on a level that really makes a difference in the world.
Learning at these levels requires more than watching videos (or lectures) and doing exercises. It takes hard work (by both the learner and the instructor), difficult assignments that get students to work at these higher levels, open channels of communication that do not just go one way, and above all a relationship between learner and instructor that engenders trust.
Maybe what’s right about the Khan Academy is that it can get all of the learning about a subject required for standardized tests out of the way when the student is not in the classroom so that they complete more difficult, higher level assignments with a trusted instructor in the classroom. I think that anyone who believes that Khan Academy is trying to replace this is severely misguided.